Referring to FIG. 1, anti-lock brake systems typically include a normally open solenoid valve 10 that is positioned between a master cylinder brake circuit 12 and a wheel caliper circuit 14 to interrupt build-up of brake fluid pressure and restrict brake fluid flow when the brakes are actuated into an anti-lock mode. When the solenoid valve 10 is in the open position, the restriction in brake fluid flow is preferably as low as possible to prevent degradation in the braking performance during normal apply and release of the brakes. However, during the anti-lock mode, it is preferred that the flow restriction in the valve 10 be high enough such that an in-rush of brake fluid, when the valve 10 switches from closed to open, is limited. The in-rush of fluid may be quantified as the pressure gain in the wheel caliper circuit. Limiting the pressure gain when the brakes are switched to the anti-lock mode is desirable to prevent excessive overshooting of the target pressure level in the wheel caliper circuit being controlled during anti-lock mode. To accomplish high flow restriction and limit pressure gain with a one size solenoid valve, either flow restriction or a reduced pressure gain must be compromised.
To reduce the problems associated with typical anti-lock braking systems, one known brake system, as depicted in FIG. 2, employs a switchable orifice 100 that cooperates with a solenoid tappet 110. The switchable orifice permits free flow of brake fluid between the master cylinder circuit 115 and the wheel caliper circuit 120 during normal braking. When the solenoid tappet 110 is energized, it closes against a valve seat 125 during the anti-lock mode. A pressure difference across a switching valve 130 overcomes a switching valve spring 135, causing the switching valve to close and restrict the flow to the switched orifice 100 which is impressed into the switching valve. The flow path between the master cylinder circuit and the wheel caliper circuit is then restricted during anti-lock mode build cycles. The flow path remains restricted until pressure is released from the master cylinder circuit, i.e., the driver releases the brake. While systems of this type allow for high flow restriction and serve to limit pressure gain, they are undesirable due to higher part complexity, increased manufacturing costs and hysteresis of the moving portion of the valve.
This system is also undesirable as the it requires the use of a Bernoulli effect force on the back side of the switching valve to prevent unwanted switching of the valve during a fast brake apply. The Bernoulli effect is generated by use of the bernoulli orifice 140. With this arrangement, the secondary orifice of the switching valve is limited in size such that this type of arrangement may only be useful in smaller vehicles. Further, systems of this type also require an additional elastomer seal 145 to prevent leakage of brake fluid while in the switched position.
Therefore, there exists a need for a switchable orifice solenoid valve for an anti-lock brake system that limits pressure gain during the anti-lock braking mode while simultaneously restricting in-rush of brake fluid during valve switching, where the system is cost effective to manufacture and eliminates hysteresis and the use of a Bernoulli effect force on the back side of the switching valve.